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002642 Although several other factors besides precipitation commonly Influence sediment yield, these either are associated with precipitation or are broadly uniform throughout the Yampa River basin. The bedrock geology of the Yampa River basin is principally Interbedded sandstone, mudstone, and shales. Likewise, hillslope relief Is generally similar throughout the basin so that neither of these factors can be primarily responsible for the observed variations in sediment yields. Conversely, soil type and ground cover vary In the. basin, but these factors are closely related to the distribution of precipitation. Increasing ground cover and soil development as precipitation Increases are primarily responsible for these decreases In sediment yield. Thus, although the potential erosion by rainfall Increases eastward. in the basin, increased soil development and ground cover more than compensate, so that sediment yield decreases with increasing precipitation. Large increases (from 60 to 500 percent) In coal mining In the Yampa River basin are projected for the next 15 years. Most of this additional coal production will be from surface mines. Even with the regulatory con- trols of the Surface Mining Control and Reclamation Act of 1977 (Public Law 95-87), substantial quantities of additional sediment are estimated to be eroded from the surface-mined areas and may be transported into the main-stem Yampa River. The potential Impact of surface mining would be the greatest In the eastern part of the basin. Existing sediment yields are less than 100 (tons/mi2)/yr or 35 (t/km2)/yr in this area, because of the extensive ground cover and soil development. Rainfall Intensities are, however, greater in the eastern part of the basin than In the western part. Consequently, one can anticipate significantly greater sediment yields In the eastern part of the basin from lands which have been disturbed by surface mining. Estimated sediment yields from surface-mined lands were computed by the PSIAC method. During mining and before complete reclamation, sediment yields are likely to be 4,000 (tons/mI2)/yr or 1,400 (t/km2)/yr. Based on these estimates and the projected ranges of volumes of mined coal, the total. amount of additional sediment supplied to the Yampa River In 1990 due to surface mining may be between 10,000 and 30,000 tons (9,ODO and 27,000 t) annually. The significance of this additional sediment load within the basin depends largely upon where the majority of the sediment enters the main-stem Yampa River. Streams in the eastern part of the basin carry. relatIvely small sediment loads under present conditions, and, hence, the additional sediment yield from surface-mined land could have a considerable Impact. For example, if all of the surface mining were located In areas of the Yampa River subbasin upstream from Hayden, the additional sediment could increase the annual load by as much as 30 percent, even though the amount of land disturbed would be less than 3 percent. As the amount of surface mining shifts to areas west of Hayden, the potential impact to the Yampa River decreas.es in relative terms. Even at the greatest projected volume of coal production, the additional sediment yields due to surface mining probably may not increase the present sediment load carried by the Yampa River near Maybell by more than an estimated 7 percent. 31 ,.